Compile the code and execute it, giving it the date of the most recent backup that was carried out. If this is the first backup you have done, put in an old date to make sure that all the files are copied. You can see this in the image below:

Done. This will generate a new subdirectory called Backup_using_FSMin the root directory, with the files and structure of the subdirectory files_need_backing_up. Next time you execute your code, it will just copy the files that have been modified since the date that you put in as a parameter and which shows the date of the last backup carried out.